Carvacrol acts as a potent selective antagonist of different types of nicotinic acetylcholine receptors and enhances the effect of monepantel in the parasitic nematode Ascaris suum

•Pyrantel, morantel and bephenium induce dose-dependent contractions of the neuromuscular flaps of A. suum.•In A. suum, carvacrol 100 μM decreased the Emax of pyrantel, morantel and bephenium with EC50 ratio of 3.43, 2.95 and 2.47.•In A. suum, carvacrol 300 u M reduced the Emax of pyrantel, morantel and bephenium with EC50 ratio of 3.88, 3.19 and 4.83.•Carvacrol enhances the inhibitory effect of monepantel on contractions of A. suum.•In the high doses/concentrations, carvacrol can interact with nicotinic receptors in mammals. The neuromuscular system of parasitic nematodes has proven to be an efficient pharmacological target for antihelmintics. Some of the most frequently used antiparasitic drugs are agonists or antagonists of nicotinic acetylcholine receptors (nAChRs). The antinematodal mechanism of action of carvacrol involves the inhibition of parasite muscle contraction. We have examined the interaction of carvacrol with antinematodal drugs that are agonists of different subtypes of nAChRs and monepantel, which is a non-competitive antagonist of this receptor in A. suum. Additionally, we investigated the effect of carvacrol on the muscle type of nAChRs in the mammalian host. As orthosteric agonists of nAChR, pyrantel, morantel and befinijum lead to dose-dependent contractions of the neuromuscular preparation of Ascaris suum. Carvacrol 100 μM decreased the Emax of pyrantel, morantel and bephenium by 29%, 39% and 12 %, 39 % and 12 % respectively. The EC50 ratio was 3.43, 2.95 and 2.47 for pyrantel, morantel and bephinium, respectively. Carvacrol 300 u μM reduces the Emax of pyrantel, morantel and bephenium by 71%, 80% and 75 %, 80 % and 75 % respectively. The EC50 ratio for pyrantel, morantel and bephenium was 3.88, 3.19 and 4.83 respectively. Furthermore, carvacrol enhances the inhibitory effect of monepantel on A. suum contractions, which may have an effective clinical application. On the other hand, tested concentrations of carvacrol did not significantly affect the EFS-induced contractions of the rat diaphragm, indicating a lack of interaction with the postsynaptic nAChR at the muscle end plate in mammals, but the highest concentration (300 μM) caused a clear tetanic fade. Carvacrol exhibited a time and dose-dependent effect on the Rota-rod performances of rats with a high value of the ED50 (421.6 mg/kg). In our research, carvacrol dominantly exhibited characteristics of a non-competitive antagonist of nAChR in A. suum, and enhances the inhibitory ef